By: Adam Gushgari, Senior Director of Emerging Contaminants, Eurofins Environment Testing
The physical particle isn't necessarily the only concern with microplastics exposure. As particles move through natural environments, they can adsorb environmental contaminants onto their surface, and they can also develop biofilms of their own. Both processes add potential routes of exposure beyond the plastic itself. This is one of the more heavily debated corners of microplastics science, and the field hasn't reached consensus on how much risk either vector actually carries.
The Chemical Vector
Adsorption is the process where atoms, ions, or molecules adhere to a surface without penetrating it. The rate depends, in part, on effective surface area – the portion of a surface actually accessible to binding. As plastics fragment down toward the nanoplastic range, total surface area increases, meaning a given mass of nanoplastics has far more binding capacity than the same mass of larger particles.
That mechanism plays out clearly with PFAS. Research has shown under realistic water conditions, PFAS adsorbs onto microplastics relatively quickly, reaching equilibrium within 7 to 9 hours, with more adsorption occurring at higher salinity and temperature and less at higher pH [1]. Weathered microplastics develop new binding sites over time, and the relationship isn't one-directional. Plastics can also desorb PFAS back into the environment, and co-exposure to PFAS and microplastics together has been shown to produce effects that are partly additive and partly synergistic, potentially more detrimental than either exposure alone [2].
Metals tell a similar story. A lab research project demonstrated that within one hour of exposure, 40 to 70 percent of several metals, including arsenic, chromium, lead, and rare earth elements, have been shown to adsorb onto plastic surfaces, with smaller particles picking up more per unit mass [3]. In a simulated gastrointestinal solution, those same metals then desorbed back off the plastic, though that finding was flagged as proof-of-concept rather than a real-world estimate.
It's important to note that not everyone agrees this adds up to meaningful risk. One influential review argued that oceanic microplastic has already reached chemical equilibrium with the surrounding water, and that plastic holds only a tiny fraction, roughly two ten-thousandths of one percent, of the ocean's total pollutant load, meaning natural diet would greatly exceed microplastic contribution to chemical exposure in most marine habitats [4]. A more recent modeling study largely agrees, with a nuance: at typical ingestion rates the contribution to human chemical exposure is negligible, but at the highest estimated exposure rates, for the most hydrophobic chemical classes, the model crossed into a safety concern for two of the four chemicals tested [5].
So the debate isn't whether microplastics can adsorb contamination. It's whether that adsorption adds meaningful risk, and the answer likely depends on the specific plastic and the specific contaminant, not a universal rule.
The Biological Vector
Biofilm development is a natural process. Microbes colonize any hospitable surface, and plastic is no exception – but what's notable is what's been observed specifically on microplastic surfaces. Research suggests that plastics may develop a biofilm that is taxonomically distinct from the surrounding water [6], and in controlled comparisons, two human pathogens and one plant pathogen have been found colonizing microplastic biofilms while remaining absent from biofilms grown on natural substrates under identical conditions [7], suggesting plastic may actively influence which bacteria take hold on it.
The antimicrobial resistance angle is where this gets more concerning. Research has shown antimicrobial-resistant bacteria have been found at 100 to 5,000 times higher concentration on microplastic surfaces than in surrounding water in some studies, and horizontal gene transfer appears to happen more efficiently inside plastic biofilms than among free-floating bacteria [6]. More recent work frames microplastics as functioning simultaneously as a microbial habitat, a resistance gene reservoir, and a transfer medium [8].
That framing has its critics, though. One widely cited review took a different approach entirely, asking not whether plastic hosts a distinct microbial community, but whether that community is actually more dangerous than what you'd find on wood, cellulose, or glass. Across genera containing known pathogens, the pattern held regardless of surface type, similar or even higher levels turned up on natural materials too. The researchers noted most colonizers appear to be opportunistic generalists, largely indifferent to what surface they happen to land on [9].
Where this debate gets genuinely interesting is at the intersection with the chemical vector above. One study measured PFAS adsorption onto microplastics in two settings side by side, a controlled lab environment and a real lake, and the gap was striking. Plastics deployed in the lake picked up PFAS at 24 to 259 times background concentration, while plastics sitting in the lab barely adsorbed any, somewhere between one-seventh and one-fourth of background. The authors attributed that gap directly to the organic matter and biofilm present in the field but absent in the lab [10].
The Bigger Picture
That finding points to a potentially compounding problem. Biofilm may not just be a biological exposure risk in its own right, it may also be a significant driver of the chemical exposure risk discussed above. The chemical and biological vectors aren't fully separate stories. They are possibly feeding into each other, and untangling exactly how is one of the more important open questions in this space.
References
[1] Salawu, Omobayo A., Christopher I. Olivares, and Adeyemi S. Adeleye. "Adsorption of PFAS onto secondary microplastics: A mechanistic study." Journal of Hazardous Materials 470 (2024): 134185.
[2] Wang, Ping, Yu-Zhen Shi, and Qingqing Guan. "The microplastic–PFAS nexus: From co-occurrence to combined toxicity in aquatic environments." Toxics 13.12 (2025): 1041.
[3] Hildebrandt, L., et al. "Microplastics as a Trojan horse for trace metals." Journal of Hazardous Materials Letters 2 (2021): 100035.
[4] Koelmans, Albert A., et al. "Microplastic as a vector for chemicals in the aquatic environment: critical review and model-supported reinterpretation of empirical studies." Environmental science & technology 50.7 (2016): 3315-3326.
[5] Gouin, Todd, and Michael Whelan. "Evaluating microplastic particles as vectors of exposure for plastic additive chemicals using a food web model." (2024).
[6] Bowley, Jake, et al. "Oceanic hitchhikers–assessing pathogen risks from marine microplastic." Trends in microbiology 29.2 (2021): 107-116.
[7] Wu, Xiaojian, et al. "Selective enrichment of bacterial pathogens by microplastic biofilm." Water research 165 (2019): 114979.
[8] Kazmi, Syed Shabi Ul Hassan, et al. "The plastisphere as a nexus for antimicrobial resistance: micro (nano) plastics in pathogen colonization, gene transfer, and global health risks." Biological Reviews 101.4 (2026): 2007-2032.
[9] Oberbeckmann, Sonja, and Matthias Labrenz. "Marine microbial assemblages on microplastics: diversity, adaptation, and role in degradation." Annual review of marine science 12.1 (2020): 209-232.
[10] Scott, John W., et al. "Perfluoroalkylated substances (PFAS) associated with microplastics in a lake environment." Toxics 9.5 (2021): 106.